A rotorcraft has a power plant for setting its rotary wing into rotation. The power plant is provided with one or more heat engines for driving the gearing of a power transmission gearbox, referred to as the main gearbox in a helicopter, said gearbox then driving the rotary wing in rotation.
The gearbox includes mechanical moving elements, such as gears and shafts, that come into contact with one another. Friction between these mechanical elements gives rise to heating that, in the long run, and if no precautions are taken tends to cause the mechanical elements to jam.
In order to avoid destroying the gearbox, it is provided with a main lubrication circuit, and advantageously with a secondary lubrication circuit. Reference may be made for example to document FR 2 826 094. The lubricant projected in the form of a jet or indeed a mist serves to cool the mechanical elements so as to limit heating thereof.
Lubrication may be performed using a lubricant of the oil type.
In a variant, it is possible to use a fluid having high latent heat suitable for absorbing a large amount of heat on changing state from a liquid state to a gaseous state. Thus, the lubricant may be pure water or a mixture that is based on water, antifreeze, and lubricant. For example, the lubricant generally known under the name glycol comprises water, 10% to 20% ethylene glycol for retarding a change from the liquid state to the solid state, 0.1% to 1% sodium sulfide, and benzothiazole-2-yl, or highly refined mineral oils. Consequently, it is possible to consider using the mixture described by the standard NF E48-602, in category HFC.
Furthermore, it should be observed that throughout the present specification, the term “heat engine” is used to cover not only turbine engines, but also piston engines.
Each heat engine is suitable for operating at a plurality of power ratings.
For example, thermal limits of a heat engine and torque limits of the gearbox enable three normal ratings to be defined for the use of the heat engine of a rotorcraft:                takeoff rating, corresponding to use that damages neither the gearbox nor the heat engine over a limited takeoff duration, usually lying in the range five minutes to thirty minutes, with this being known as maximum takeoff power (TOP);        maximum continuous power corresponding to use that damages neither the gearbox nor the heat engine during unlimited use, with this being referred to as maximum continuous power (MCP); and        maximum transient rating, possibly limited by regulation: this is referred to as maximum transient power (MTP).        
There also exist higher power contingency ratings for multi-engined rotorcraft, that are used in the event of one heat engine being inoperative (OEI):                the first contingency rating during which the capabilities of the gearbox on its inlet stages and the thermal capabilities of the heat engine are used to the maximum: this may also be referred to as super-emergency power (PSU) or 30-sec OEI since it can be used for a maximum of thirty consecutive seconds, and at least three times during a flight; if 30-sec OEI is used, then it may be necessary to remove and overhaul the heat engine;        the second contingency rating in which the capabilities of the gearbox concerning its inlet stages and the capabilities of the heat engine are used very largely: this is referred to as maximum emergency power (PMU) or two min OEI since it can be used for two minutes after thirty sec OEI or for two minutes and thirty seconds consequently, at most; and        the third contingency rating during which the capabilities of the gearbox concerning its inlet stages and the thermal capabilities of the heat engine are used without damaging them: this is referred to as intermediate emergency power (PIU) or MCP OEI that can be used for thirty minutes or continuously for the remainder of the flight after a heat engine has become inoperative.        
Nevertheless, the power developed by the heat engine at any particular rating may be found to be insufficient in special circumstances, e.g. in a hot atmosphere.
For example, the 30-sec OEI rating may be sufficient to allow the pilot to land the aircraft, while being insufficient to enable the mission to be terminated. In addition, the length of time this super-emergency power can used may be only just sufficient or may even be insufficient to guarantee aircraft safety.
It is known to inject pure water or a mixture of water and alcohol into a turbine engine in order to increase the power it develops without changing the temperature at the outlet from the combustion chamber. For example, with a free turbine engine, pure water or such a mixture is injected in order to increase the power developed without increasing the temperature of the gas at the outlet from the combustion chamber, where said temperature is referred to as the “T4 temperature” both by the person skilled in the art, and below in the present specification.
Pure water injection was used in piston engines during World War II and has subsequently been used on turbojet airplanes.
Water may be injected into the combustion chamber of the heat engine. This changes the composition of the gas generated by the gas generator. This results in a change in the heat capacity and in the specific enthalpy (enthalpy per unit mass) of the gas. For unchanging flow rate of gas created in the combustion chamber, the power generated by the heat engine increases with increasing specific enthalpy of the gas that is created.
Under such circumstances, in a turbine engine, injecting water enables gas to be expanded at greater specific enthalpy at constant temperature T4, while also increasing the flow rate of air through the turbine. The power developed by the turbine engine is thus increased.
Nevertheless, the fuel flow rate is increased at constant temperature T4 in the combustion chamber, insofar as the energy absorbed by vaporizing water in the combustion chamber needs to be compensated by delivering additional heat.
When the heat engine is a turbine engine having a gas generator with an air inlet and a compressor upstream from a combustion chamber, the water may be injected into the air inlet.
Under such circumstances, the total mass flow rate through the air inlet is increased, thereby delivering additional power at constant specific enthalpy. The power available at constant temperature T4 and at constant air flow rate is thus increased.
Nevertheless, the fuel flow rate is also increased for constant temperature and constant air flow rate.
It should be observed that unlike the above circumstance, the operating point of the compressor is modified. In addition, the compressor generally needs to work harder in order to deliver the energy needed for evaporating the liquid water.
Water may be injected in the form of a mist. In order for the method to be advantageous, it is necessary for the mist to be sufficiently fine to enable the water to evaporate prior to entering into the compressor. Under such circumstances, the temperature at the inlet to the compressor decreases, thus enabling higher overall efficiency to be obtained and a greater mass flow rate. Nevertheless, the compressor must deliver more work insofar as the mass flow rate is greater. Furthermore, the operating point of the compressor is modified, with some of the surplus enthalpy that is generated serving to satisfy this additional need for power.
In addition, although the gain in power is positive, it is not obvious, a priori, that the efficiency of the turbine will be better when water is injected. Injecting water requires a higher fuel flow rate and that leads to higher specific consumption.
Furthermore, and above all, the use of the technique of injecting a water-based fluid requires a device to be implemented that has a reserve containing said fluid and a control member that manages the injection of the fluid. That device consequently increases the weight of the power plant. The gains achieved under certain circumstances by such a device are potentially limited or indeed completely lost by the increase in weight associated with the presence of an additional device.